The European Commission has issued a fresh warning that it will not reopen the Brexit Withdrawal Agreement as Theresa May prepared for further talks in Brussels.
he UK Prime Minister will meet commission president Jean-Claude Juncker in the Belgian capital on Wednesday evening, commission chief spokesman Margaritis Schinas announced.
The meeting follows talks on Monday between the the EU's chief negotiator, Michel Barnier, and Brexit Secretary Stephen Barclay and Attorney General Geoffrey Cox.
Afterwards, Mr Barclay described their discussions on the issue of the Northern Ireland backstop - which remains the main stumbling block to a deal - as "productive".
At the daily briefing for journalists in Brussels, Mr Schinas said the EU side wanted to achieve an "orderly" withdrawal by the UK on March 29 as planned.
However, he made clear that the EU was not prepared to give ground on Mrs May's key demands - either to include an exit clause or a time limit on the backstop or to replace it with an alternative arrangement.
"The EU will not reopen the Withdrawal Agreement. We cannot accept a time limit to the backstop or a unilateral exit clause," he said.
"Further talks will be held this week to see whether a way through can be found that would gain the broadest possible support in the UK Parliament and respect the guidelines agreed by the European Council.
"We are listening and working with the UK government to see how we can work for an orderly withdrawal of the United Kingdom from the European Union on March 29. That is where we are."
With a fresh series of House of Commons votes expected next week, Mrs May is looking for concessions that will enable her to get her deal through Parliament after it was comprehensively rejected by MPs in last month's "meaningful vote".
Despite the continuing impasse over the backstop - intended to ensure there is no return of a hard border on the island of Ireland - ministers have again insisted there will be no delay to Brexit and the UK will leave on March 29 as planned.
However, Mr Juncker said he could not rule out an extension to the Article 50 withdrawal process which would keep the UK in the EU beyond the elections to the European Parliament due in the spring.
In an interview with the German newspaper Stuttgarter Zeitung, he said no-one on the EU side would oppose an extension to Article 50.
On the prospect of the UK electing MEPs to the new Parliament, he said such a scenario was "difficult to imagine", adding that it would be a "belated joke of history" but he did not rule it out.
However, arriving for a meeting of EU foreign ministers in Brussels, Brexit Minister Lord Callanan said Mrs May was determined the UK should leave as planned.
"The Prime Minister has been very clear that we intend to leave on March 29, that's what Article 50 says and that's what our domestic legislation says," he said.
Lord Callanan said the EU had acknowledged the backstop was "only ever intended to be temporary" and that the UK was "seeking to explore with them how we can codify that in a legally-binding way that is acceptable to our Parliament".
The Department for Exiting the EU (DExEU) confirmed the discussions with Mr Barnier on Monday included the so-called "Malthouse compromise" being worked on by Tory MPs in the Alternative Arrangements Working Group.
However, there appeared to be little enthusiasm for the proposals - intended to replace the backstop with a basic free trade deal combined with technological solutions to avoid the need for physical border checks - on the EU side.
"While the commission engaged seriously with these proposals, it expressed concerns about their viability to resolve the backstop," a DExEU spokesman said.